[Amps] 8877 grid current going negative etc

[almacottage]

hi all.

i wonder if someone would have any ideas as to an intermittent occurrence i see on one of my 8877 6m amplifiers .

i have one running at 4.5kv off load which is seeing the problem occur but im not sure if its tube related or something else.

if i am running into dummy load and have it with 50w drive and 2kw output the amp may decide to all of a sudden go negative on the grid current and rf output is lost .
at the same time there will be 500ma of plate current reading ["stuck on"] without the PTT engaged .
it may or may not drop off after a few seconds .

after this it can do one of 2 things , either it will again make grid current go negative and the plate current will stay up on 500ma with no rf output , or it will again return to making rf output .

at the same time as the amp does this the input swr goes sky high also .

i can switch amp off and then run through warmup cycle and it may well run ok again or it may suddenly go as it did previously .

i have ran it up to 2.5kw output for 70w drive and it can be  all ok for a while or it can play up .

running it at 1500w output seems fine but i have not ran it long enough to see a problem occur yet .

anyone got any ideas as to why this may occur, what it might be, and where i should look for the cause .

the amp is same design as by geoff brown  GJ4ICD , although uses vacuum caps in the plate and load tuning with a flat strip plate 3 turn  x 1 1/4" plate coil and the usual  input tuning of 2 coils 10t each with 50pf variable in series and a 3600pf cap at the input connector  .

PSU is separate to rf deck ,  3.2kv transformer @ 1.2amp - 30uf  6kv oil/paper cap - 47k bleeder resistors .

biasing is via an 18v zener diode and runs at 80ma standing current .

 i did have a 12v zener in there but bias standing current was 200ma and so changed for the 18v one .

the amp has shown same symptoms with both zeners fitted .

i am trying to figure out of tube related or possibly something else .
 i have a spare 8877 tube i can try , that i know is a working tube .

what id like to find out is  how and why the grid current is suddenly shooting to negative and then directly after the anode/plate current is stuck on 500ma with the PTT not engaged  , and input swr is infinate when the grid current shoots negative .

would it be an oscillation/feedback problem , or at a long shot a problem with something happening at a certain stage of tuning .

i get no arcs or sparks when this happens .

plate DC blocking cap is 20kv 1000pf , it uses no anode parasitic choke, i wondered if it may be worth having one in there but ive not seen any 8877 6m amps using one anyway .

ive not really tested it for a long period of time each time but suffice to say amp will run ok at times and then perhaps problem will occurr, but it can be switched off again, then back on and will run ok for a while .

hope someone can work something out from the details i have written .

thanks .

ian